why in c++ there are no compile-time error if class field is not initialized?

Question

In my class I was using such field:

private:
bool firstSeqNumReceived;

Everything were working fine for a while but after one commit I've figured out that now field is true by default. Surprisingly field is not initialized to false by default, instead assigned value is implementation dependent (refer to What is the default value for C++ class members for more details )

Now I wonder why compiler doesn't produce compile-time error forcing me to add initialization? Who needs "implementation-dependent" default value, are there any use-cases? Why not produce compile-time error in this case?

If you know your code is going to assign a value before you read it, you don't need initialization. :) — Joe, Apr 15 '13 at 11:25
One of the ideas of C++ is that you pay for what you need. If you don't need to initialize the variable, the compiler doesn't force the overhead on you. — Shahbaz, Apr 15 '13 at 11:26
Efficiency. Construction of object is about obtaining the memory and run constructor initialization list and constructor code. If the user desires to set the values later you can do that. — Abhijit-K, Apr 15 '13 at 11:29
Technically, the value isn't implementation-dependent, it's *unspecified*, and using the value gives undefined behaviour. That's especially the case for `bool`, where the uninitialised value might be neither `true` nor `false`. — Mike Seymour, Apr 15 '13 at 11:41

score 5 · Accepted Answer · answered Apr 15 '13 at 11:27

I wonder why compiler doesn't produce compile-time error forcing me to add initialization?

Because the compiler assumes you know what you're doing.

In C++ you don't pay for what you don't use. There may be use cases where initialization is a waste of time. For example, in a class where real meaningful values for members cannot possibly be computed until after the object has been constructed, initializing the members to some default or sentinel value accomplishes little.

score 0 · Answer 2 · answered Apr 15 '13 at 11:25

0

The use-case is basically, that it depends on the program flow if you are using the variable or not. If you don't use it in parts of your code then there is also no need to zero initialise it.

Also if you know, that the value will be assigned in the constructor or some other member function it would be just a superfluous step to first write 0 into memory.

answered Apr 15 '13 at 11:25

filmor

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who needs variable that never initialized and never used? – Oleg Vazhnev Apr 15 '13 at 11:26
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@javapowered, example: `int x; scanf("%d", &x);`. Here, `x` doesn't need initialization. – Shahbaz Apr 15 '13 at 11:27
@javapowered A data member may be used in only a certain branch of control flow. If it is assigned a value on this control flow before it's read, there's no problem with leaving it uninitialised in other control flows. – Angew is no longer proud of SO Apr 15 '13 at 11:42

score 0 · Answer 3 · answered Apr 15 '13 at 11:26

The standard specifies that it is undefined behaviour to access an uninitialized object. There are a few reasons such undefined behaviour appears in the standard. This one is more about not giving you more than you ask for. Sometimes you really don't want to initialize a variable as soon as you create it. Instead, the compiler trusts you to initialize an object when you need it initialized. As an example of when you don't want it initialized automatically:

int x;
std::cin >> x;

It would be completely pointless to automatically initialize x to 0 as the value is immediately overwritten. The power to initialize a variable is in your capable hands.

Compilers may also assume you do not invoke undefined behaviour in order to make optimizations.

why in c++ there are no compile-time error if class field is not initialized?

3 Answers3